In the photographic arts, hydrophilic colloids, typically refined gelatin, have been used in photographic elements to form layers, such as radiation-sensitive layers (e.g., silver halide emulsion layers), interlayers, subbing layers and overcoat layers. Various addenda are conventionally incorporated in the layers, including addenda to alter the properties of the hydrophilic colloids present therein. It has been recognized previously in the art that without special modifiers, hydrophilic colloids such as gelatin are easily abraded and ingest large quantities of water when brought into contact with aqueous solutions, thereby causing an undesirable amount of swelling. Also, unmodified colloid coatings tend to melt at relatively low temperatures, thereby limiting their temperature range of utility. To obviate these deficiencies, certain addenda generically designated as "hardeners" are incorporated into hydrophilic colloids intended to be used in forming photographic element layers. Both inorganic and organic hardeners are known. A summary of hardeners is presented in Research Disclosure, Item 36544, September, 1994. The terms "forehardened" and "forehardener" are employed when the hardener is associated with hydrophilic colloid in the course of manufacturing a photographic element. The terms "prehardened" and "prehardener" are employed when the hardener is associated with a hydrophilic colloid layer of a photographic element in a processing solution preceding the developer bath for the element.
Among hardeners of the active olefin type, a preferred class of hardeners particularly useful as forehardeners are compounds comprising two or more vinyl sulfonyl groups. These compounds are hereinafter referred to as "vinyl sulfones." Compounds of this type are described in numerous patents including, for example, U.S. Pat. Nos. 3,490,911, 3,642,486, 3,841,872 and 4,171,976. Vinyl sulfone hardeners are believed to be effective as hardeners as a result of their ability to crosslink polymers making up the colloid. One disadvantage that has been observed in using vinyl sulfone hardeners is that these hardeners, particularly the more active ones, such as bis(vinylsulfonyl)methane (BVSM), will homopolymerize. Homopolymerization can occur before the hardeners are associated with the hydrophilic colloid and it can occur as a competing reaction after association. The tendency of the hardeners to homopolymerize is disadvantageous in requiring careful selection of hardener preparation and handling conditions and in causing hardener to become unavailable for the desired crosslinking reaction with the hydrophilic colloid. The consequences of such homopolymerization include the inability to maintain the desired concentration of hardener in the hardening solution and the plugging of delivery lines and other equipment with the polymer that is formed, thereby necessitating frequent downtime for cleaning and maintenance.
The problem of homopolymerization of vinyl sulfones used in compositions for hardening of photographic gelatin has long been recognized in the photographic art. Thus, for example, D. M. Burness et al in U.S. Pat. No. 4,171,976, issued Oct. 23, 1979, describes this problem and discloses the use of certain inhibitors such as 3,5-dinitrobenzoic acid to prevent or minimize homopolymerization of bis(vinylsulfonyl)alkane hardeners. Similarly, Burness et al U.S. Pat. No. 3,841,872, issued Oct. 15, 1974, utilizes hydroquinone as a stabilizer in the preparation of bis(vinylsulfonyl)alkane hardeners.
In the preparation and handling of hardening solutions containing a vinyl sulfone hardening agent, it is advantageous to utilize a conductivity marker, i.e., a material that is added to the relatively non-conductive hardening solution to assist with identification by rendering it feasible to monitor electrical conductivity and thereby ensure delivery of the correct hardener solution to the coating station. Thus, for example, a desirable conductivity for a BVSM solution is 2.6 to 3.0 mhos/cm. The conductivity marker should not adversely affect the quality or handling of the hardening solution.
A compound that has been commonly used as a conductivity marker in photographic hardening compositions is potassium nitrate. However, the use of potassium nitrate is disadvantageous in that it contributes to and accelerates the formation of homopolymer in vinyl sulfone hardening solutions when ferric ion is present in levels of only a few parts per million. Such levels of ferric ion are commonly encountered, with a major source being the iron oxides formed on the surfaces of iron pipes and other equipment used to convey the hardening solution and a minor source being iron present in the vinyl sulfone monomer from its synthesis. Thus, when using potassium nitrate as a conductivity marker, it is difficult to meet required quality specifications and the homopolymerization which occurs results in clogged delivery lines and frequent interruptions in the photographic manufacturing process.
In light of the above, it is apparent that the photographic art is in need of a highly effective stabilizer for vinyl sulfone hardening compositions that is of low cost, free from adverse sensitometric effects, highly effective in inhibiting homopolymerization in the presence of ferric ion and useful as a conductivity marker. It is toward the objective of providing such new and improved stabilizer that the present invention is directed.